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Full text of "Text Book Of Mechanical Engineering"

Copying Principle.

139

feed) a plane surface;, and nothing could be more satisfactory.
If this be the completion of the cycle, as we suppose, then the
reciprocating tools, with lost back-stroke, must ultimately give
way.

The Copying Principle is another great principle involved
in both hand and machine tools. All depend for their accuracy
on one or more carefully-prepared copies contained within the tool.
Thus in the carpenter's chisel the flat back is held against the
wood when paring, and constitutes the copy. The sole of a hand
plane serves the same purpose, its truth or otherwise beirjg copied
on the work, which may be proved by curving the sole, and thus
obtaining curved surfaces.

The copying principle is universal. Take the lathe : the bed
has a plane surface truly parallel to the line of centres, thus
enabling us to produce a true cylinder as our solid of revolution.
A second slide at right angles to the former gives us a
copy for use in (surfacing/ producing plane ends or right
cylinders.

The V grooves of the planing machine give accuracy along the
table, while the cross beam or slide ensures truth across it, and so
we obtain a true plane. The vertical slide and the two horizontal
cross slides are the copies in the slotting machine, while the
shaping machine hdis two copies supplied by the horizontal slides,
at right angles. Lastly, the milling machine has two slides, at
right angles and also horizontal.

As the truth or otherwise of these copies is transferred to the
work, it is of the utmost importance that they should be made
perfectly correct in the first instance.

The copying latfce and other duplex wood-working machines
are farther examples of the principle. (See Appendix II,,
p 812.)

/ Cutting Tools.óWe will "now consider the shapes and
angles required for the tool itself. As a rule wood-working tools
act by wedging, or splitting-off the sharing; and the resistance is
tensile, with some bending. Our interest is with cutting tools for
rnetal, and Prof. R. H. Smith has shewn their action to be totally
different.

Tlie diagram Fig. 129 represents the tool in action,    B is the